Can I buy an iphone off someone and put my sim card in it?

I want to buy an iphone and put my sim card into it. Will it work? I have At&t as my provider. Also, I don't have internet on my phone. If I get the iphone would at&t charge me for internet or anything extra?

Yes, you can. As long as you have a sim card you can put it into an iPhone. I don't think At&t will charge you if you don't have internet but a lot of apps involve internet. So it will be best if you just add it to your plan. Plus it's cheap. But if not, then all you can use your phone for is texting, calling, music, and the camera.